This invention generally relates to a varistor and, more particularly, relates to a varistor which has two spaced electrodes attached to its surface and insulated from each other by a high resistivity surface layer.
Varistors are extensively used both as arrestors, which conduct unusual and high voltages to the ground in order to protect an electrical system from the voltage, and as surge absorbers, which absorb surge, such as switching surge, because of their highly nonlinear resistance, which varies inversely with the applied voltage. A typical varistor includes a sintered body which contains zinc oxide (ZnO) as a major component and small amounts of one or more additional metal oxides, such as bismuth trioxide (Bi.sub.2 O.sub.3), antimony trioxide (Sb.sub.2 O.sub.3), cobalt (III) oxide (Co.sub.2 O.sub.3), manganese (II) monoxide (MnO) and chromium (III) sesquioxide, (Cr.sub.2 O.sub.3). A pair of spaced electrodes is provided on the surface of the body. The sintered body is prepared by mixing the additional metal oxide with zinc oxide, granulating the mixture, forming a granulated powder, and sintering. This prior art varistor has a highly nonlinear characteristic compared with the older silicon carbide (SiC) varistor. It is believed that the improvement in the nonlinear characteristic is due to the interface between zinc oxide particles in the sintered body and the boundary layer surrounding the zinc oxide particles. The boundary layer consists of the additional metal oxide. Such a varistor (zinc oxide) also has the property that the nonlinearity may be adjusted to some extent by selecting the kind and amount of additional metal oxide.
The prior art zinc oxide varistor explained above is unsuitable for use as a power arrestor under circumstances in which a high voltage (such as 1 MV) will be applied to it. Specifically, the nonlinear resistance characteristic of such a varistor, which does not have any coating on its surface, is unstable in high ambient humidity because the sintered body of the varistor tends to absorb moisture. Moreover, after a high impulse current flows through the varistor, there is a large change in the resistivity of the varistor. Consequently, a varistor without any coating on its surface is not suitable for use as an overvoltage protection device, such as an arrestor or surge absorber, to which lightning pulses and surge voltage pulses may be applied for a long time.
It is generally required that a varistor have the following characteristics in order to perform satisfactorily as an overvoltage protection device:
(1) The nonlinear resistance characteristic of the varsitor must be unaffected, or hardly affected, by ambient conditions, such as humidity. That is, the varistor should have a stable nonlinear characteristic.
(2) The resistivity value of the varistor must not change, or must change very little, after a high impulse current is applied to it.
(3) The varistor must have an extremely small leakage current flowing on the surface of the sintered body when a high voltage is applied. This property enables the varistor to tolerate a large peak current.
In order to use the zinc oxide varistor as an overvoltage protection device, it has been proposed that the exposed surface of the sintered body be coated with a layer of epoxy resin. However, a varistor with an epoxy resin layer cannot tolerate large peak currents.
It has also been proposed, in U.S. Pat. Nos. 3,872,582, issued Mar. 25, 1975, 3,905,006, issued Sept. 9, 1975, and 4,031,498, issued June 21, 1977, that a high resistivity layer comprising zinc orthosilicate (Zn.sub.2 SiO.sub.4) and/or zinc antimonate (V) (Zn.sub.7 Sb.sub.2 O.sub.12) be provided on the exposed surface of the sintered body. Although a varistor with such a high resistivity layer has both an improved tolerance to large peak currents, and a more stable characteristic in high humidity, compared with the epoxy resin coated varistor, the resistivity stability requirement is not fully satisfied when such a varistor is used as an arrestor.